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The purpose of the 12 volt battery is to provide power to boot up the computers so the car can be "READY" for driving. Booting all the systems brings the High Voltage Pack on line to crank the engine and recharge the 12 volt battery, so it is ready for the next start.

Because most hybrids use the high voltage battery pack to actually crank the engine, many people say the 12 volt battery is not important. They couldn't be more misinformed. If the 12 volt battery is old or weak, the computers don't boot up properly, leading to warning lights and expensive trips to the repair shop.

In fact, a weak 12 V battery will mess with the memory of the battery management computer and can cause a HYBRID PACK FAILURE! We are starting to see this on a pretty wide scale on Priuses.

Some batteries are conventional and can be replaced by a generic battery from the auto parts store or the big box automotive department. (We still don't recommend cheap batteries, though...) Other hybrids use special batteries, which are physically different, making it hard to substitute a generic battery in. Most Pruises fall into this category.

PRIUS 12 VOLT BATTERIES

Prius 12 volt batteries are troublesome for several reasons. They are a special battery both in size, shape, connections, and chemistry, and they are pricey. We have looked for alternatives and found nothing that makes straying from Toyota replacement parts a reasonable choice . (With the exception of Gen 1, below.)

Another major issue with Prius batteries is that they test OK when they are actually not. So reliable testing is not possible. The decision to replace a Prius battery needs to be made by age, and even earlier if any symptoms show up. Symptoms of a bad battery may include having to jump start the car but that is less likely. More likely is a warning light or two, and random codes in some of the computers. We have been able to save many new customers from a code based part replacement by doing a second opinion on a diagnostic and finding a bad 12 V battery! For Pruis models the 12 Volt battery should be replaced every four years-no questions asked.

From the beginning, Toyota has made two battery sizes available for most of the Prius models. If we are replacing a battery and we encounter a smaller battery, we will always upgrade to the larger size. This is a Minnesota thing. The cold weather is just too hard on the smaller versions. To install the larger battery, we buy a kit or make a modification to the battery tray to accommodate the larger footprint. Once installed, you cannot tell it has been changed (except for the size, of course).

Gen 1 priuses ('01 thru '03) also had two sizes. To replace the smaller one with a new larger one, requires a kit from Toyota to make the factory installation . Most have been converted by now to the bigger size, but we can still get the kit to make the big battery fit (2015). We also have found a high quality batttery which we use in place of the Toyota battery, to save significant cost. This is the only Toyota model which we will replace with a non Toyota (factory/ OE) part. It is also the only Prius battery that we can reliably test after it has been in service a few years.

OTHER BATTERY ISSUES

Hondas tend to have problems that look like battery problems, but come back soon after a battery replacement. We have found that this usually is the result of poor connections in the power supply circuits. For us, it is not a big deal to find and fix- it's just what we do. A good interview and history of the car lets us know we need to do a simple test for this. And another dead give away is a history of having to replace the battery every two years or so.